The present invention relates to methods and materials for the fabrication of micro-electronic circuits and particularly to the fabrication of insulating layers therein. It is particularly advantageous in the fabrication of multi-layer micro-electronic circuits.
Multi-layer micro-electronic circuits are built up on insulating substrates by superimposing printed conductive ink patterns and layers of insulating material. The layers of insulating material are usually glaze compositions, that is to say powdered glasses or mixtures of powdered glasses, and may be deposited in a suitable binder or medium, for instance by spraying or spinning processes. The binder is evaporated or decomposed into volatile products which are driven off, and the glaze subsequently fused by heating.
There are many requirements to be satisfied in the choice of a suitable glaze composition. The glaze must be chemically compatible with the surface on which it is to be deposited, so as to provide a strongly adherent and reliable bond. It must be physically compatible, having similar thermal expansion properties to the substrate and not too low a thermal conductivity, so as to resist thermal shock in manufacture and use. It must have a high electrical resistivity, preferably greater than 10.sup.11 ohms per square, to avoid short-circuiting and deleterious capacitive coupling effects on the circuit. It should also have a sufficiently high thermal conductivity to allow for the dissipation of heat from the microcircuit without the development of excessive temperatures or temperature gradients. It must be fusible at a convenient temperature not likely to damage or distort any previously deposited parts of the circuit, so as to form a reliable, continuous, non-porous, stable and adherent insulating layer.
When several layers of conductive networks are required they must be separated by insulating layers, and it is clearly necessary to fuse each insulating layer without causing any flow or distortion in previously deposited layers; for this reason it has generally been considered necessary to develop and use a series of mutually compatible metallic inks and glaze compositions having progressively lower fusion temperatures, in order to make multilayer circuits. In order to allow a convenient tolerance on the temperatures of successive heat-treatments, and to ensure that fusion of each layer could be achieved without distortion of the preceding layer, it was considered desirable that there should preferably be a temperature difference of at least 150.degree.C. or more between any pair of consecutive heat-treatments.
For reasons of convenience, expense, and safety, and also to avoid the decomposition of some constituents of the glaze compositions, it is preferable to use only materials which can be heated to fusion temperatures in an oxidising atmosphere, for instance air. This restricts the choice of metals usable for printing the conductive patterns.
In view of all these restrictions, it will be readily understood that it is difficult to find an ideal series of materials to facilitate the fabrication of microelectronic circuits of more than a few layers. The use of a series of different materials, and the requirement for heat treatments at a plurality of accurately controlled temperatures, are experimentally and commercially inconvenient. The use of materials which do not quite satisfy all the requirements, or inaccuracies in the temperature controls tend to reduce the yield of satisfactory circuits.